The new research, which was published on March 2nd in the journal Nature Geoscience, used the quality of hydrothermal chemistry to show that the surface of Earth was most probably covered by an ocean about 3.2 billion years ago.
Gathering Clues From Soil
The research team’s discoveries could help experts better understand the way and were single-cell organisms first appeared on Earth, as per Boswell Wing, a paper co-author.
“The history of life on Earth tracks available niches,” said Wing, an associate professor in the Department of Geological Sciences at the University of Colorado Boulder. “If you’ve got a water world, a world covered by ocean, then dry niches are just not going to be available.”
The research also stirs a long-debated issue over what ancient Earth may have looked like in terms of temperature.
“There was seemingly no way forward on that debate,” said lead author Benjamin Johnson, who led the research throughout a postdoctoral position in Wing’s lab at CU Boulder. “We thought that trying something different might be a good idea.”
For Johnson and Wing, that ‘something different’ focused on a geologic site known as the Panorama district, located in Northwestern Australia’s wilderness.
“Today, there are these really scrubby and rolling hills that are cut through by dry river beds,” said Johnson, now an assistant professor at Iowa State University in Ames. “It’s a crazy place.”
It is also the place in which a 3.2 billion-year-old hump of ocean crust that’s been turned on its side can be found. The scientists saw the crust covering the place as a unique opportunity to gather clues about the chemistry of ocean water from many billion years ago.
“There are no samples of really ancient ocean water lying around, but we do have rocks that interacted with that seawater and remembered that interaction,” Johnson said.
The process required a thorough and challenging analysis, but the team examined data from over 100 rock samples from all over the dry terrain.
Most of the Planet Was Water
The team was looking, in particular, for two distinct isotopes, also explained as flavors of oxygen caught in stone – a heavier atom known as Oxygen-18 and a lighter one called Oxygen-16.
The researchers found that the proportions of those two isotopes of oxygen may have been trapped in seawater 3.2 billion years ago, as they had a bit more Oxygen-18 atoms than what you’d find today.
“Though these mass differences seem small, they are super sensitive,” Wing said.
It turns out, however, that the differences are only sensitive to the presence of continents. Wing explained that the current land masses are coated with soils rich in clay that irregularly take out heavier oxygen isotopes from the water like the Oxygen-18 atoms.
The team hypothesized that the most probable explanation for that abundance of Oxygen-18 in the early oceans was that there were no soil-rich continents at that time to take up the isotopes. However, that doesn’t mean that there was no dry land at all.
“There’s nothing in what we’ve done that says you can’t have teeny, micro-continents sticking out of the oceans,” Wing said. “We just don’t think that there was a global-scale formation of continental soils like we have today.”
Therefore, when did plate tectonics move to form the continents of today’s Earth? Researchers are not sure of that, but they are planning to analyze other, younger rock samples to see if they can determine when the landmasses first appeared.